In recent years, along with the popularization of personal computers and others, a digital still camera and a digital video camera (which, are simply called “a digital camera” from flow on) which can easily take image information into digital equipment are spreading over a level of an individual user. The digital cameras of this kind are expected to spread more and more as an input equipment, also in the future.
Further, downsizing of a solid-state image pickup element such as CCD (Charge Coupled Device) mounted in a digital camera is advanced. Along with the advancement of downsizing, the digital cameras are required to be further downsized. For this reason, an image pickup lens system that occupies the greatest volume in the digital input equipment is strongly requested to be compact. The easiest way to make an image pickup lens to be downsized is a method to make a size of a solid-state image pickup element to be small. For that purpose, however, it is necessary to make a size of a light-receiving element to be small, which requires a higher level of difficulty of manufacturing a solid-state image pickup element and requires higher capability of the image pickup lens.
On the other hand, when a size of an image pickup lens is made small while keeping a size of a solid-state image pickup element as it is, a position of an exit pupil gets closer inevitably. When the position of an exit pupil gets closer, an off-axis light flux emitted from an image pickup lens enters an image surface obliquely. Therefore, light-converging capability of a micro-lens is hardly exhibited sufficiently, and it causes a problem that brightness of an image is extremely uneven between a central portion and a peripheral portion of an image. When trying to keep a position of an exit pupil of an image pickup lens at a distance for solving the aforesaid problem, size growing of a total image taking lens is not avoidable. In addition, a demand of low-cost of an image pickup lens has become strong, because of competition to keep the prices down in recent years. Further, density growth of a solid-state image pickup element in recent years has raised capability requested for the image pickup lens.
For the aforesaid demands, there has been formulated a compact lens in a three-lens structure for a solid-state image pickup element (for example, see Patent Document 1: Unexamined Japanese Patent Application Publication No. 2004-309695).